1. Field of the Invention
The present invention relates to a heat-sensitive recording paper and, more particularly, to a heat-sensitive recording paper capable of forming a dye image having improved preservability under high temperature and high humidity conditions.
2. Description of the Prior Art
In heat-sensitive recording papers images are formed utilizing physical or chemical changes in the materials induced by heat energy, and a large number of processes using heat-sensitive recording papers have been investigated.
One type of heat sensitive recording paper which relies upon a heat induced physical change is a so-called wax type heat-sensitive recording paper which has been well known and used for recording electrocardiograms and the like. On the other hand, as a material utilizing a chemical change due to heat to form images, a number of materials based on various different color-forming mechanisms have been proposed. Of these, a binary coloration system is most typical.
A binary coloration system is prepared by dispersing two heat-reactive compounds as fine particles in a binder or the like such that the binder separates them from each other, and coating the resultant mixture on a base paper. The thus prepared paper records images by heating it to melt one or both of the compounds such that they come into contact with each other, thus causing a color-forming reaction. These two heat-reactive compounds are generally referred to as electron donating compounds and electron accepting compounds, respectively. An extremely large number of combinations of them are known, and they are roughly classified as systems forming images of a metal compound and systems forming dye images.
Representative examples of systems forming images of a metal compound are those in which the electron donating compounds are organic reducing agents such as phenols, chelating agents, sulfur compounds, or amino compounds and the electron accepting compounds are organic metal salts. These two produce a metal, metal complex compound, metal sulfide, or the like through the reaction therebetween upon being heated, thus providing a colored image. Specifically, there are a combination of thiourea and heavy metal salt (U.S. Pat. No. 2,740,895), a combination of gallic acid or the like and metal salt of stearic acid (U.S. Pat. Nos. 2,663,654, 2,663,655, 2,663,656, and 2,663,657), a combination of hydroquinone and silver behenate (U.S. Pat. No. 3,031,329), a combination of hexamethylenetetramine and tin compound U.S. Pat. No. 2,813,043), and the like.
On the other hand, examples of systems forming dye images are those in which the electron donating compound is an electron donating colorless dye, and the electron accepting compound is an acidic material such as phenol or the like (Japanese Patent Publication No. 4,160/68 and U.S. Pat. No. 3,451,338).
The binary coloration systems of heat-sensitive recording papers have many advantages as recording papers. For example, (1) they are based on primary coloration and do not require development processing: (2) the paper quality is approximate to that of ordinary paper; and (3) they are handled with ease. In particular, those materials wherein a colorless dye is used as the electron donating compound have the additional advantages that (4) they provide higher color density and (5) they enable one to easily obtain heat-sensitive recording papers forming different colors, thus being more valuable. Therefore, they are in the most use as heat-sensitive recording papers.
As the electron donating colorless dyes, triarylmethane compounds, diphenylmethane compounds, xanthene compounds, thiazine compounds, spiropyran compounds, and the like have been used. Several examples thereof are illustrated below. As the triarylmethane compounds, there are 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (or crystal violet lactone), 3,3-bis(p-dimethylaminophenyl)-phthalide, 3-(p-dimethylaminophenyl)-3-(1,3-dimethylindol-3-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl)phthalide, etc. As the diphenylmethane compounds, there are 4,4'-bis-dimethylaminobenzhydrin benzyl ether, N-halophenyl-leucoauramine, N-2,4,5-trichlorophenylleucoauramine, etc. As the xanthene compounds, there are rhodamine-B-anilinolactam, rhodamine-B-(p-nitroanilino)lactam, Rhodamine-B-(p-chloroanilino)lactam, 3-diethylamino-7-(dibenzylamino)fluoran, 3-diethylamino-7-phenylaminofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-(3,4-dichloroanilino)fluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-diethylamino-7-phenylfluoran, etc. As the thiazine compounds, there are benzoyl leucomethylene blue, p-nitrobenzyl leucomethylene blue, etc. As the spiro compounds, there are 3-methyl-spiro-dinaphthopyran, 3-ethyl-spiro-dinaphthopyran, 3,3'-dichloro-spiro-dinaphthopyran, 3-benzyl-spiro-dinaphthopyran, 3-methyl-naphtho-(3-methoxybenzo)-spiropyran, 3-propyl-spiro-dibenzopyran, etc. They may be used alone or in combination.
Specific examples of conventional electron accepting compounds include phenol compounds, organic acids or metal salts thereof, hydroxybenzoic acid esters, etc. Of these compounds, phenol compounds are favorably used because they have a melting point which is near the desired recording temperature (70.degree. to 120.degree. C.) and they do not require the use of a low-melting compound or, if any, in only a small amount. They are described in detail in, for example, Japanese Patent Publication No. 29,830/76, and U.S. Pat. 3,539,375. To be specific, there are illustrated 4-t-butylphenol, 4-phenylphenol, 4-hydroxydiphenoxide, .alpha.-naphthol, .beta.-naphthol, methyl-4-hydroxybenzoate, 2,2'-dihyroxybiphenyl, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), 4,4'-isopropylidenebis(2-chlorophenol), 4,4'-isopropylidenebis(2-methylphenol), 4,4'-sec-isobutylidenediphenol, etc. However, these phenol compounds are not necessarily considered to be satisfactory heat-sensitive materials.
That is, frequently they suffer from one of the following defects: (1) inadequate color density is obtained when used in combination with an electron donating colorless dye; (2) fog (i.e., color formation during storage before use) tends to be formed; (3) the color fastness of the images formed is not sufficient. Specifically, 4,4'-isopropylidenediphenol (2,2-bis(4-hydroxyphenyl)propane) most generally used as the phenol compound tends to cause fog, though it provides the highest color density of the above-described electron accepting compounds. When Crystal Violet Lactone is used as an electron donating colorless dye, the phenol compound causes fog during storage under high temperature and high humidity (45.degree. C., 80% RH) conditions and serious fading of a colored images occurs.
In addition to the foregoing, in order to obtain sufficient color density at a color-forming temperature of about 100.degree. C., a melting point-reducing agent must be used together with the phenol compound due to the high melting point of the phenol compound (156.degree. C.). The heat-sensitive recording sheets, particularly those presently used in facsimile recorders, are required to be recorded at recording temperatures of about 80.degree. to 120.degree. C., and hence too high melting points are not preferable.